What are the laws of thermodynamics?

Thermodynamics is the science of heat, work and temperature. It rests on four short laws, and they explain everything from an ice cube in your drink to the engine in a car.

Temperature is how fast molecules jiggle. In air at 20 °C the molecules fly at about 500 metres a second, some slower, some faster. Hot air's molecules are faster. The board shows their speeds: the curve is the Maxwell–Boltzmann spread.

Zeroth law: put two things in touch and heat flows until they reach the same temperature. That is thermal equilibrium, and it is why a thermometer works: it settles to the temperature of whatever it touches.

First law: energy is never made or destroyed (see EnergyClear). For a gas it reads ΔU = Q − W: the change in the gas's inner energy U equals the heat Q put in, minus the work W the gas does pushing the piston. Switch to Piston and watch the books balance.

Second law: heat flows from hot to cold by itself, never the other way. The reason is counting. There are vastly more ways for energy and molecules to be spread out than bunched up, so things spread. The measure of that spreading is entropy, and in anything left alone it only grows. Third law: you can get ever closer to absolute zero, −273.15 °C, but never reach it (see chapter 5).

A heat engine turns heat into motion. Fuel burns and makes a hot gas, the hot gas pushes, and the push drives wheels or a generator. Car engines, motorbikes, power stations and jet engines are all heat engines.

In 1824 a young French engineer, Sadi Carnot, worked out the rule for all of them. Heat can only do work while it flows from something hot to something cold, just as a water wheel needs water to fall. And some heat must always be dumped into the cold side. You can never turn all of it into work.

The best any engine can possibly do is the Carnot limit: efficiency = 1 − T_cold ÷ T_hot, with both temperatures in kelvin. Between 600 K and 300 K, at most half the heat can become work. That is the second law in its engineer's form: the entropy the heat carries in must be carried out, and the cold side can only take it with some heat.

A car's petrol engine squeezes air and fuel into a small space, sparks it, and the burning gas at about 2,500 K pushes the piston down. Carnot would allow 88%. Real engines manage about 25 to 35%: heat soaks into the cylinder walls and coolant, the exhaust leaves still very hot, and friction takes its share. See CarClear and MotorcycleClear.

Heat flows from hot to cold by itself. So on a 35 °C day, heat leaks into a 24 °C room through the walls, the window and the roof, all the time. To keep the room cool, something has to carry that heat back out, uphill, from cold to hot.

That machine is a heat pump. An AC and a fridge are both heat pumps. A cold liquid in the inside coil soaks up heat, the compressor squeezes the vapour until it is hotter than the outside air, and the outside coil dumps the heat there. See ACClear and FridgeClear.

The second law says moving heat uphill always costs work. The first law says where it all goes: heat out = heat taken + electricity. So the outdoor unit blows out more heat than the AC removes from your room.

How much heat you move per unit of electricity is the COP (coefficient of performance). Carnot's rule caps it at T_cold ÷ (T_hot − T_cold), in kelvin. The smaller the temperature gap, the cheaper the pumping. That is why setting your AC at 24 °C instead of 18 °C saves so much.

Drop ice into a warm drink and the drink gets cold. It feels as if cold flows out of the ice. It doesn't. Heat flows out of the drink into the ice, downhill, as the second law says. There is no such thing as a flow of "cold".

That heat first warms the ice from the freezer's −18 °C up to 0 °C. Then it does something sneaky: it melts the ice without warming it at all. Melting 1 gram takes 334 joules, enough to cool 80 grams of drink by 1 °C. That hidden latent heat, not the ice's coldness, is why ice cools a drink so well.

At the end, the drink and the meltwater sit at one temperature. That's the zeroth law again: thermal equilibrium. And the total entropy has gone up: the ice gained more entropy than the drink lost, because the same heat counts for more at a lower temperature (ΔS = Q ÷ T).

On the hob, heat tumbles down a staircase: a flame at about 1,950 °C, the pot, the water at up to 100 °C, and finally the kitchen at 30 °C. Every step makes entropy, and none can run backwards. Hot flue gas that misses the pot goes up the chimney hood. See ChimneyClear, and AirFryerClear for heat blown into food.

Third law: as things get colder, molecules slow down, and at absolute zero, 0 K or −273.15 °C, they would have the least motion that quantum physics allows. But every cooling step removes only part of the heat that is left, so you can get closer and closer and never arrive. Walther Nernst worked this out between 1906 and 1912.

Physicists have come astonishingly close. With laser beams that push on atoms and then let the fastest escape, they cooled rubidium to 170 billionths of a kelvin in 1995, and the atoms merged into one quantum blob, a Bose–Einstein condensate. Deep space, by comparison, is a warm 2.7 K.

Maxwell's demon is a thought experiment from 1867. A tiny being opens a door only for fast molecules going one way and slow ones going the other. The gas splits into hot and cold with no work: the second law broken? The catch, found by Rolf Landauer and Charles Bennett, is information. The demon must remember each molecule, and wiping its memory costs at least k T ln 2 of heat per bit, which makes more entropy than it removed.

Life seems to break the second law too: a seed builds a tree, beautifully ordered. But living things take in low-entropy food or sunlight and give out heat and waste with far more entropy. The total still grows. Order here is paid for by more disorder elsewhere.

Myth-buster: a fridge does not "make cold". It moves heat. Open its door in a closed kitchen and the room gets warmer, by exactly the electricity it uses. And the second law is about odds, not force: a gas could un-spread by chance, but for real numbers of molecules the chance is so tiny it never happens in the life of the universe.